Objective: To define the added value of three-dimensional transvaginal ultrasound (3D TVUS) and 3D gel infusion sonography (GIS) in the diagnostic work-up of patients with a uterine septum by determining the sensitivity, specificity and interrater reproducibility. Design: Prospective multicentre cohort study Patients: Patients aged 18-45 years with suspicion of a uterine septum on two-dimensional (2D) TVUS, without prior septum surgery Intervention: 3D TVUS ± GIS Comparison: Pelvic MRI Outcome: The primary outcome is the sensitivity of 3D TVUS. The secondary outcomes are the specificity of 3D TVUS, the sensitivity and specificity of 3D TVUS with GIS, the interrater reproducibility of 3D TVUS and 3D GIS for the diagnosis of a uterine septum, and patient experience in performing these examinations. Finally, cost-effectiveness of 3D TVUS (± GIS) will be evaluated.